SQL data manipulation language

SELECT statement

How many PC models does a particular supplier produce? How the average price is defined for computers with the same specifications? The answers to these and other questions associated with some statistic information may be obtained by means of summarizing (aggregate) functions. The following aggregate functions are assumed as standard:

All these functions return a single value. In so doing, the functions COUNT, MIN, and MAX are applicable to any data types, while the functions SUM and AVG are only used with numeric data types. The difference between the functions COUNT(*) and COUNT(<column name>) is that the second does not calculate NULL values (as do other aggregate functions).

Example 5.5.1

Find out the minimal and maximal prices for PCs:

Console

Execute

SELECT MIN(price) AS Min_price, MAX(price) AS Max_price
FROM PC;

SELECT MIN(price) AS Min_price, MAX(price) AS Max_price FROM PC;

    Scheme

The result is a single row containing the aggregate values:

Example 5.5.2

Find out the number of available computers produced by the maker А:

Console

Execute

SELECT COUNT(*) AS Qty
FROM PC
WHERE model IN(SELECT model
 FROM Product
 WHERE maker = 'A'
 );

SELECT COUNT(*) AS Qty FROM PC WHERE model IN(SELECT model FROM Product WHERE maker = 'A' );

    Scheme

As a result we get:

Example 5.5.3

If the number of different models produced by the maker A is needed, the query may be written as follows (taking into account the fact that each model in the Product table is shown once):

Console

Execute

SELECT COUNT(model) AS Qty_model
FROM Product
WHERE maker = 'A';

SELECT COUNT(model) AS Qty_model FROM Product WHERE maker = 'A';

    Scheme

Example 5.5.4

Find the number of available different PC models produced by maker A.

This query is similar to the preceding one for the total number of models produced by maker A. Now we need to find the number of different models in the PC table (available for sale).

To use only unique values in calculating the statistic, the parameter DISTINCT with an aggregate function argument may be used. ALL is another (default) parameter and assumes that all the column values returned (besides NULLs) are calculated. The statement

Console

Execute

SELECT COUNT(DISTINCT model) AS Qty
FROM PC
WHERE model IN (SELECT model
 FROM Product
 WHERE maker = 'A'
 );

SELECT COUNT(DISTINCT model) AS Qty FROM PC WHERE model IN (SELECT model FROM Product WHERE maker = 'A' );

    Scheme

gives the following result:

If we need the number of PC models produced by each maker, we will need to use the GROUP BY clause, placed immediately after the WHERE clause, if any.

Suggested exercises: 10, 11, 12, 13, 18, 24, 25, 26, 27, 40, 41, 43, 51, 53, 54, 58, 61, 62, 75, 77, 79, 80, 81, 85, 86, 88, 91, 92, 93, 94, 95, 96, 103, 109, 127, 129

Content:

(5.5.1.) GROUP BY clause
(5.5.2.) HAVING clause
Summarizing data using ROLLUP
Combination of detailed and aggregated data
The sorting and NULL values
Aggregate function to aggregate function
Product of column values